Optical transmission systems employ Wavelength Division Multiplexing (WDM) to increase information handling of an optical fiber transmission line, typically a long haul transmission line. Early WDM systems operated with a relatively narrow wavelength bandwidth, centered around 1550 nanometers, e.g. 1530-1565 nanometers, referred to as the C-band. This is the wavelength region where standard silica based optical fibers have optimally low absorption.
In most WDM systems there is a trade-off between the number of channels the system accommodates and the channel separation. Higher bit rates generally call for an increase in channel spacing. Both goals favor a wide operating spectrum, i.e. a wide range of operating wavelengths.
Recently, systems have been designed that extend the effective operating wavelength range well above the C-band transmission band. In terms of wavelength, the new band, referred to as the L-band, is variously defined, but for the purpose of this description is 1570-1610 nanometers. Substantial work has also been done in the S-band, defined as 1460-1530 nm. Use of these added wavelengths substantially extends the capacity of WDM systems. There is an ongoing effort to further extend the effective operating wavelength window to above 1610 nm, for example to 1620 nm. Success of these efforts will depend on finding components, for example amplifiers, that provide effective operation over this broad wavelength range. It is now well appreciated that a transmission fiber should have a minimum level of dispersion at signal wavelengths to enable WDM transmission by suppressing four wave mixing impairments. Since the dispersion of a non-zero dispersion-shifted NZDF typically increases toward longer wavelength, this requirement implies that the zero dispersion wavelength should be 20-40 nanometers lower than the shortest wavelength intended for WDM.
In WDM systems, it is important to have uniform gain over the entire WDM wavelength band. This objective becomes more difficult to reach as the operating wavelength range is extended to longer and/or shorter wavelengths. Recently, new types of optical fiber amplifiers have been developed that operate using stimulated Raman scattering. The most prominent of these is a distributed amplifier that operates over the normal transmission span as a traveling wave amplifier. Raman scattering is a process by which light incident on a medium is converted to light at a lower frequency (Stokes case) than the incident light. The pump photons excite the molecular vibrations of the medium up to a virtual level (non-resonant state). The molecular state quickly decays to a lower energy level emitting a signal photon in the process. Because the pump photon is excited to a virtual level Raman gain can occur for a pump source at any wavelength. The difference in energy between the pump and signal photons is dissipated by the molecular vibrations of the host material. These vibrational levels determine the frequency shift and shape of the Raman gain curve. The frequency (or wavelength) difference between the pump and the signal photon is called the Stokes shift. In Ge-doped silica fibers, the Stokes shift at which the maximum gain is obtained is ˜13 THz. Due to the amorphous nature of silica the Raman gain curve is fairly broad in optical fibers.
Since Raman scattering can occur at any wavelength, this can be exploited to advantage in a telecommunication system that contains multiple signal wavelengths by using Raman pumps at several different wavelengths to amplify the signals. The gain seen by a given wavelength is the superposition of the gain provided by all the pumps, taking into account the transfer of energy between the pumps due to Raman scattering. By properly weighting the power provided at each of the Raman pump wavelengths it is possible to obtain a signal gain versus wavelength profile in which there is a small difference between the gain seen by different signal wavelengths (this difference is called the gain ripple or gain flatness). The use of Raman amplification thus enables dense WDM (DWDM) outside the erbium window. Raman amplification is also an enabling technology for the evolution from 10 to 40 Gb/s transmission because it improves optical signal to noise ratio at lower launch powers.
A multiplicity of pumps has been used successfully in many systems. However there is one persistent problem with multiple pumps. An unwanted nonlinear effect called four-wave mixing (FWM) may sometimes occur. In telecommunications systems, if FWM occurs in the signal band this can lead to transmission errors. As the number of pumps in a multi-pump wavelength Raman amplification scheme increases, the likelihood of FWM increases.
The harmful effects of four-wave mixing have been recognized. Recently one approach towards reducing these effects has been proposed [EP 1 148 666 A2]. In this approach the pump wavelengths are either time division multiplexed (TDM) together, or the frequency of the pump source is modulated (FM). Since the various pump wavelengths overlap for only a small distances along the fiber, FWM between the pump wavelengths should be eliminated or severely reduced.
While this approach would eliminate FWM, the nominal pump power requirements in this system are relatively high. Moreover, to TDM a relatively large number of pump wavelengths, some operating at relatively high power, adds significantly to the cost and complexity of the system. Other approaches to reducing FWM and other non-linear effects would significantly advance the art.
At least equally as important as compatibility with amplifier technology in the design of optical fibers for high bit rate, wide-band, systems is management of chromatic dispersion. This problem grows significantly as the data bit rate is increased. An optical transmission line, comprising a cabled fiber and a dispersion compensation element (typically a module but possibly a cabled fiber), that transmits effectively at 10 Gb/s may show excessive error rates at 40 Gb/s because of bit overlap. For non-return-to-zero modulation, a 10 Gb/s system should accumulate less than ˜1000 ps/nm chromatic dispersion over the total link distance; for a 40 Gb/s system this requirement is tightened to less than 60 ps/nm.
This requirement is met by a combination of two methods. First, in NZDF fibers, dispersion is reduced in the C-band below that of standard matched clad fiber. To gain this benefit over multiple bands, it is advantageous that the slope of the dispersion be low. Second, dispersion compensation technology is employed, most commonly in the form of a dispersion compensating fiber (DCF) in a module. For broadband operation, it is important that the dispersion curve of the DCF “match” that of the transmission fiber in the appropriate sense. In general, precise compensation of chromatic dispersion over a broad band is achieved when the ratio of the dispersion slope to the dispersion at band center is equal for the fiber and DCF. Furthermore, the best results are obtained when this ratio is low. This further emphasizes the advantage of reduced dispersion slope.
A problem arises in designing optical fibers to meet this general need: typical optical fiber profiles that are optimized for low dispersion slope have reduced effective area due to bend loss constraints. Optical fibers with reduced effective area generally show increased and unwanted non-linear effects including four-wave mixing as well as self- and cross-phase modulation (SPM, XPM). For Raman amplified systems, too small effective area exacerbates the issues of “Raman gain tilt” whereby shorter wavelength pumps (signals) transfer energy to longer wavelength pumps (signals).
Thus the manufacture of optical fibers for high bit rate (e.g. 40 Gb/s) systems and with both low dispersion slope and medium or large effective area, while at the same time preserving other performance characteristics such as low Polarization Mode Dispersion (PMD), is a design challenge.